[Intel-gfx] [PATCH v2] drm/i915: use effective_size for ringbuffer calculations

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              364/364              363/364
ILK              +1-1              364/366              364/366
SNB                 -1              448/450              447/450
IVB                 -1              497/498              496/498
BYT                 -1              289/289              288/289
HSW                 -1              563/564              562/564
BDW                 -1              416/417              415/417
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 PNV  igt_gem_exec_blt      NRUN(6, M23M7M25)PASS(1, M25)      NRUN(1, M7)
 ILK  igt_gem_exec_blt      NRUN(7, M37M26)PASS(1, M26)      NRUN(1, M37)
 ILK  igt_kms_flip_bcs-flip-vs-modeset-interruptible      DMESG_WARN(1, M26)PASS(10, M37M26)      PASS(1, M37)
 SNB  igt_gem_exec_blt      NRUN(7, M35M22)PASS(1, M35)      NRUN(1, M22)
 IVB  igt_gem_exec_blt      NRUN(7, M34M21M4)PASS(1, M34)      NRUN(1, M34)
 BYT  igt_gem_exec_blt      NRUN(7, M48M49M50M51)PASS(1, M48)      NRUN(1, M50)
 HSW  igt_gem_exec_blt      NRUN(7, M40M20M19)PASS(1, M40)      NRUN(1, M19)
 BDW  igt_gem_exec_blt      NRUN(7, M30M28)PASS(1, M28)      NRUN(1, M28)
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